18 DOUBLE MONSTROSITY— STRUCTURE, CLASSES II. AND III. 



resulting vessel is next joined by the third and fourth aortic roots, but it remains separate from its 

 fellow on the other side, so long as the notochords are separate, i.e. back to the twentieth somite. 



The arrangement of vessels which has just been described and is illustrated in PI. XVIII. fig. 68, 

 is somewhat remarkable. Mixing of arterial with venous blood must have taken place in no slight 

 degree, and certain parts in each twin head must have been supplied by blood coming directly from the 

 ventral aorta. There is no trace of a pair of adjacent jugular veins, and it is difficult to make out from 

 sections the course of the venous blood coming from the adjacent sides of the twin heads. But as 

 there is a considerable amount of spongy tissue below the base of the skull and in the septum 

 between the mouth-openings, it is probable that the blood in question found its way into the median 

 and the main jugulars. The presence of this spongy tissue no doubt indicates congestion. 



Kidneys. The pronephric glomerulus is composite. It is remarkably large, and is divided into 

 three compartments by two vascular tufts each of which has an afferent and an efferent vessel. 

 Normal Wolffian ducts arise from the outer compartments, while the middle compartment gives 

 origin to a sacculated tubule which passes backwards a short distance to end blindly, and 

 represents fused adjacent Wolffian ducts. 



Alimentary Canal. There are two buccal cavities, but the oesophagus and the rest of the 

 alimentary tract are single except for the presence of two air-bladder diverticula. 



Summary for Class II. Union of the simple lateral type occurring at the hind-brain ; 

 notochords further apart in the cervical than in the cranial region, and uniting from the 20th 

 to the 30th body-segment; body thereafter having a normal bilateral structure; skeletal elements 

 in floor of cranium doubled from parachordal region forwards ; very little doubling in the visceral 

 arch skeleton ; greater duplicity in the floor than in the roof of the medulla and first portion of the 

 spinal cord ; all four eyes separate ; inner auditory organs present but reduced ; heart normal ; 

 vessels and pronephros greatly modified ; mouth partly double but pharynx and gut single ; two air- 

 bladder diverticula. 



The chief uniting and composite structures noted are : the parachordal, palato-quadrate, 

 Meckelian, hyoid, supraorbital, auditory, neural, and haemal arch cartilages ; the medulla and first part 

 of the spinal cord ; the ventral aorta and various large vessels ; inner muscle plates ; pronephros. 



That secondary fusion has played an important part in the ultimate moulding of the transitional 

 region in this type is evidenced by a set of characters almost exactly comparable with those referred 

 to in the description of Class I. (p. 15). What is said on p. 4 and p. 11 should also be referred to 

 in this connection. 



CLASS III. 



Union in Pectoral Region, the Inner or Adjacent Pectoral Fins not being represented} 



PI. II. figs. 7, 8 (external appearance); Pis. XV.-XVI. figs. 52-55 (horizontal sections); 

 diagrams of various structures in PL XVII. fig. 63, XIX. fig. 75, XX. fig. 87. 



Structures completely doubled. The brains, organs of sense, cranial skeletons, and anterior portions 

 of the vertebral column are completely double and separate, as also are the two visceral arch 

 systems, except for union on the part of the dorsal ends of the inner or adjacent last, and some- 

 times second last, branchial cartilages. 



The general appearance is illustrated in PI. II. fig. 7. It will be seen that the twin bodies do 

 not meet in perfectly simple lateral union, but tend to come together first by their ventral aspect. 

 This tendency, however, is not strongly marked, and is easily coiTected at the transitional region, 

 behind which the body has a normal bilateral structure. 



Notochords, etc. The notochords and spinal cords are still widely separate opposite the pectoral 

 region. They remain separate to near the 32nd body-segment, but ultimately fuse, so that the 

 posterior part of the body and the tail contain a single notochord and a single spinal cord. The 

 union of the spinal cords is well in advance of that of the notochords. 



1 See footnote on p. 11. 



